is it possible to replace face of a humanoid model with a plane face object?
Thank you for stating your problem clearly.
Yes, there is a way to do the lazy way (but will only get you half way there).
Position your FLAT face directly in front of your ALIEN head.
Select your flat face and press CTRL+3 this will give it a lot of subdivisions.
APPLY this modifier to the FLAT face.

Now, press CTRL+A to APPLY all your transformations both for the alien and the flat face.
Select the FLAT face, go to properties>modifiers>Deform>Shrink wrap. 
Select the ALIEN FACE as the object deformer and play with the X, Y, Z orientation parameters (by default should work immediately on Y).

Now you have conformed your increased polygonal flat face onto the surface of the alien's face like you wanted.

Thank you for voting me up as the answer for this question.

BFCT_Schiller